Electromagnetic switch construction

ABSTRACT

A magnetically operated switch construction which uses as its moving contact-actuating armature, a resilient circular discshaped diaphragm. The diaphragm is moved axially to actuate its contacts. The disc has three or more annularly aligned tongues, each having a face end resiliently bearing against an adjacent surface to maintain the diaphragm in its normal condition.

United States Patent Inventor Harry Stanley Woodhead Harlow, England Appl. No. 41,333 Filed May 28, 1970 Patented Dec. 21, 1971 Assignee International Standard Electric Corporation New York, N.Y. Priority June 6, 1969 Great Britain 28,697/69 ELECTROMAGNETIC SWITCH CONSTRUCTION 6 Claims, 18 Drawing Figs.

U.S. Cl 335/196 Int.Cl H0lh 1/06 Field 01 Search 335/78, 154, 179, 188, 196; 337/89 Primary Examiner-I-Iarold Broome Attorneys-C. Cornell Remsen, Jr., Walter J. Baum, Paul W.

Hemminger, Charles L. Johnson, .lr., James B. Raden, Delbert P. Warner and Marvin M. Chaban ABSTRACT: A magnetically operated switch construction which uses as its moving contact-actuating armature, a resilient circular disc-shaped diaphragm. The diaphragm is moved axially to actuate its contacts. The disc has three or more annularly aligned tongues, each having a face end resiliently bearing against an adjacent surface to maintain the diaphragm in its normal condition.

PATENTED DEE21 |97| $529 749 SHEET 1 [IF 8 pew/e Aer Fla/0,2 427' lnvenlor Harry Stanley Woodhcad J2 07 y MW 641% Attorney PATENTEU DEEZI 19?:

SHEET 3 BF 8 PATENTEU 015821 1971 SHEET 8 [IF 8 86 W JA ELECTROMAGNETIC SWITCH CONSTRUCTION BACKGROUNDOF THE INVENTION The use of a diaphragm as the moving contact of a contact unit is known and has been previously disclosed in my earlier British Pat. Nos. 1,094,334; 1,098,145; 1,099,081; 1,115,401; 1,131,928;

A problem encountered in the design of a small diaphragm for such use is set by the conflicting requirements of sturdiness and low spring rate. A low spring rate is desirable so that it can be operated by a relatively small change of magnetic flux providing a correspondingly small change of magnetic force of attraction. On the other hand, the sensitivity of the unit is impaired if the diaphragm is not substantial enough to handle without saturation the flux required to provide or overcome the necessary contact pressure for satisfactory operation.

- SUMMARYOFTI IE INVENTION It is therefore an object of the invention to provide an improved diaphragm armature, electromagnetic switch.

It is a still further object of the invention to provide an improved diaphragm armature design for use in an electromagnetic switch or relay.

It is a still further object of the invention to provide a resilient magnetic armature having a plurality of biasing and positioning members within a peripheral annulus of the diaphragm.

It is a still further'object of the invention to provide a magnetic diaphragm having an internal magnetic path productive of superior response characteristics.

These and other objects, features and advantages of the invention will become apparent from the description of the preferred embodiment of the invention and viewed in connection with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a contact diaphragm known in the art.

FIG. 2a is a schematic perspective view of the magnetic effect achieved using the diaphragm of FIG. 1.

FIG. 2b is a schematic perspective view of the magnetic effect achieved using the diaphragm of FIG. 3.

FIG. 3 is a perspective view of the diaphragm embodying the present invention.

FIGS. 4, 5, and 6 are respectively longitudinal sections through make break and changeover versions of contact units using the present invention.

FIG. 7 is a section view through an alternative form of fixed contact.

FIG. 8a is a sectional view through a relay using the diaphragm of FIG. 3.

FIG. 8b is a sectional view through an alternative embodiment of relay.

FIGS. 9, 10a, and 10b are sectional views through a further alternative embodiment of relay.

FIGS. II, l2, 13, I4, and are sectional views through different embodiments of pushbutton switches using this invention.

DESCRIPTION OF PRIOR ART The earlier specifications referred to earlier disclose how, by the cutting substantially circumferential slots in a diaphragm, its spring rate can be substantially reduced. Such a diaphragm is illustrated in FIG. 1, which shows how three slots 10 serve to define a central region 11 of the diaphragm which is linked with a peripheral region 12 by means of three tongues 13. The reduction in the spring rate produced by these slots is dependent upon the length and narrowness of the tongues they define, but there are practical limits imposed on the choice of both of these parameters. Thus the minimum width which can be tolerated is set at least partly by the risk of any of the tongues sustaining damage resulting in a permanent strain;

and while the length of the tongues can be increased by modifying the shape of the slots so that the tongues have a substantially spiral form instead of a simple arcuate one, a limit is set on their length by the need to provide an adequate area for the central region of the diaphragm.

The minimum area required for the central region of the diaphragm is in part dependent upon the magnitude of the magnetic attractive force required to act upon it. A magnetic field entering or leaving one of the plane surfaces of the region exerts an attractive force upon it proportional to the product of the square of the flux density of that field with the area over which it acts. If the flux enters the region by one side and leaves by the other there will be two forces acting in opposition, and therefore if the flux densities are the same on both sides there will be no resultant force. Consequently in order to producean attractive force upon the central region of the diaphragm it has to distribute the flux radially within its thickness so that the flux entering from one side does not all pass straight through without deviation and leave by the other. Hence, the magnetic force of attraction which can be exerted upon the central region of the diaphragm is related to its ability to achieve radial distribution of the flux. Such ability is however limited by the onset of saturation.

It will now be shown with reference to two examples illustrated with reference to FIGS. 2a and 2b that this onset of saturation is dependent not only upon the total amount and density of flux entering or leaving the diaphragm, but also upon the configuration of the area over which it enters or leaves.

In FIG. 2a the magnetic flux is shown entering the central region 20 of a diaphragm at a uniform density distributed over the surface of a circle 21. Inside the diaphragm the flux spreads out radially. It can be verified that with this distribution of flux the place of maximum flux density within the diaphragm is the curved surfaceindicated at 22 and defined by the projection of the circumference of the circle 21 through the thickness of the diaphragm. Any increase in the value of the flux entering the diaphragm under these conditions results in a larger attractive force. But once the condition is reached which produces saturation at the curved surface 22, any further increase results in a smaller increase in the force of attraction than that which would occur in the absence of saturation. This arises because the additional flux is constrained to pass straight through the diaphragm.

Similar considerations apply with the configuration illustrated with reference to FIG. 2b. In this configuration the flux also enters the diaphragm at a uniform flux density but in this case the configuration of the area over which it enters is an annulus 23 equal in area to the circle 21 of FIG. 2a. Again, it can be verified that with this distribution of flux the place of maximum flux density within the diaphragm is a curved surface, only in this instance it is the curved surface 24 defined by the projection of the outer circumference of the annulus 23 through the thickness of the diaphragm. Since however the outer circumference ofthe annulus 23 is longer than the circumference of the circle 21, the total surface area of the curved surface 24 is greater than that of the surface 22. Therefore, with the annular configuration of FIG. 2b a larger amount of flux can be applied without causing saturation than is possible with the circular configuration of FIG. 2a Stated in an alternative way, this means that the same magnetic force of attraction can be achieved with the same total magnetic flux, but with a thinner diaphragm, if a pole piece with an annular pole face is substituted for one with a circular pole face having the same area as the annular one. As a corollary to this, the bigger the outer circumference of the annular pole face, the greater is the permissible reduction in thickness of the diaphragm. Hence, for any given diameter of diaphragm of the general type illustrated in FIG. 1, the longer the tongues 13 are made the smaller is the area of the central region of the diaphragm. The smaller this area, the smaller is the area of the pole face of the pole piece that can be used with it. The smaller that this pole face is, the thicker the diaphragm has to be made in order to handle the flux required to provide the same magnetic force of attraction. Consequently, there is an optimum length for the tongues 13, beyond which the reduction in spring rate afforded by a greater length is more than offset by the increase in spring rate engendered by the requirement to make the diaphragm of thicker material.

Referring again to FiG. 1 it will be observed that if the central region of the diaphragm is displaced perpendicularly out of the plane of the peripheral region the tongues 13 act as double cantilever systems by virtue of the fact that their extremities are constrained to lie in parallel planes. Therefore, the spring rate can be further reduced by modifying the construction so as to remove this constraint and so allow each tongue to act as a single cantilever. This constraint may be removed by adopting the construction disclosed in the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT in general, there are three different basic types of contact unit; the make unit, the break unit and the changeover unit; the operation of each unit being effected by an increase in magnetic flux threading the unit. The moving contact of each of these types of unit is provided by the same basic shape of diaphragm illustrated in FIG. 3 constructed from resilient magnetic material. The diaphragm consists of a disc 30 from whose periphery protrude three tongues 31 which extend substantially circumferentially in the same plane as the disc but are bent over at their free ends to form U-shaped end portions 32 substantially at right angles to the plane of the disc 30 and protruding beneath it. The diaphragm may be made of cobalt iron, and either or both sides of it may be faced with nonmagnetic material such as copper, gold, or silver cadmium oxide to improve its electrical conductivity and its contact making properties. The facing may be applied subsequent to manufacture of the diaphragm, or alternatively the diaphragm can be out straight from bimetal or trimetal sheeting of which one of the metals is magnetic.

The configuration of the tongues is such that they can handle very little flux, and consequently the majority of any flux entering the disc 30 through either face must leave again either by the same face or by the other. if the flux leaves by the same face the attractive force on the disc is augmented, but if it leaves by the other it is diminished. in the latter case the efficiency is not seriously impaired provided that the geometry can be arranged so that the flux on one side is much more concentrated than on the other.

A make contact unit will now be described with reference to FIG. 4 which shows diagrammatically a longitudinal section through the unit. A mild steel rod 40, providing the inner magnetic member, is sealed by means of a glass-to-metal seal 41 within the central aperture of a mild steel annulus 42, providing the outer magnetic member. These components form one wall of a housing which is completed by a mild steel cap 43, which contains a diaphragm of the type described with reference to FlG. 3, the disc being depicted at 30 and the U- shaped end portions of the tongues being depicted at 32. The cap 43 is conveniently secured to the annulus 42 by the method of projection welding. inside the housing the end face of the rod 40 is bored to provide an annular pole face 44, which also serves as the fixed contact cooperating with the diaphragm.

The dimensions of these components are chosen so that while the diaphragm is not stressed the U-shaped end portions 32 of the tongues of the diaphragm serve as spacers which can bear against an internal flange 45 of the annulus 42 and so prevent the disc from coming into accidental contact with the rod 40.

Operation of the contact unit is effected by energizing a magnetic circuit so as to cause an increase, either from zero or from a remanent value, in the amount of magnetic flux threading the series combination of the rod 40 the disc 30 and the annulus 42. Although the majority of this flux also threads the cap 43, a resultant force of attraction exists between the disc 30 and the pole face 44 because in bridging this gap the flux is much more concentrated than where it bridges the gap between the disc 30 and the cap 43. The force of attraction is transmitted through the tongues of the diaphragm to their U- shaped end portions, which in consequence bear against the flange 45 of the annulus 42. As the disc 30 is attracted into contact with the pole face 44 the-tongues of the diaphragm take up the strain, each acting as a single cantilever by virtue of the fact that its U-shaped end portion is able to pivot through a limited are by rolling along the surface of the flange 45. Since the pole face 44 also serves as the fixed contact it is preferably faced with another metal such as copper, gold, silver, or silver cadmium oxide in order to improve its contact making properties. This facing, together with one on the underside of the diaphragm, also serves as a spacer preventing the two magnetic parts from coming into intimate contact.

A break contact unit will now be described with reference to FIG. 5. This contact unit is very similar to the make unit described with reference to FIG. 4, the. principal difference being that it is constructed so that an increase of flux threading the unit is required to separate' the contacts instead of being required to bring them together. This difference of function accounts for the three following differences of components used in the construction. Firstly, the seal 41 by which the rod 40 is secured within the central aperture of the annulus 42 is not necessarily electrically insulating because the rod does not form the fixed contact. it does however, still have to be of nonmagnetic material so that it will not provide an unduly low reluctance path shunting the diaphragm, and therefore it is preferred to make the seal 41, in this instance, of a nonmagnetic metal such as brass. Secondly the cap 43 is dispensed with, and its place taken by a different construction consisting of an electrically conducting rod 50 forming an inner cap member sealed within the central aperture of an annular outer cap member 52 by means of an electrically insulating seal 51 such as glass-to-metal seal. And thirdly the upper surface of the diaphragm is faced with material to improve its contact-making properties. I

The fixed contact of this contact unit is provided by the end face of the inner cap member 50 in which a shallow bore is made to provide an annular contact surface 54. Preferably this surface is faced with a metal layer designed to improve its contact-making properties.

The dimensions of the components of this contact unit are chosen so that when the outer cap member 52 is secured to the annulus 42, preferably by the method of projection welding, the fixed contact is caused to bear down upon the disc 30 so as to stress the tongues of the diaphragm and provide the necessary contact pressure when the unit is not operated. in FIG. 5 these tongues are depicted diagrammatically at 31. The contact unit is operated by energizing a magnetic circuit so as to cause an increase, either from zero or from a remanent value, in the amount of magnetic flux threading the serial combination of the rod 40 the disc 30 and the annulus 42 in order to attract the disc 30 towards the rod 40 and separate it from the fixed contact. in this construction the majority of the flux entering the underside of the disc 30 leaves by the same side so that there are two magnetic forces of attraction acting in parallel upon the disc, that due to the flux bridging the gap between the disc 30 and the pole face 44, and that due to the flux bridging the gap between the disc 30 and the annulus 42.

A changeover contact unit will now be described with reference to FIG. 6. This unit combines most of the principal features of the other two units described above the reference to FIGS. 4 and 5. Thus, it has a mild steel rod 40, which provides one of the fixed contacts, sealed by means of a glass-tometal seal 41 within the central aperture of a mild steel annulus 42. The other fixed contact is provided by the annular end surface 54 of an electrically conducting inner cap member 50 which is secured by a glass-to-metal seal 51 in the central aperture of an annular outer cap member 52.

The dimensions of this contact unit are chosen so that when the outer cap member 52 is secured to the annulus 42, preferably by the method of projection welding, the fixed contact provided by the annular surface 54 is caused to bear down upon the disc 30 so as to stress the tongues 31 and provide the necessary contact pressure between it and the disc when the contact unit is not operated. The contact unit is operated by energizing a magnetic circuit so as to cause an increase, either from zero or from a remanent value, in the amount of magnetic flux threading the series combination of the rod 40 the disc 30 and the annulus-42 in order to attract the disc 30 towards the pole face 44, first separating it from the contact face 54 and then bringing it into contact with the pole face 44. In'this construction the outer magnetic member is provided with a yoke 67 ofmagnetic material so that the majority of the flux entering the underside of the disc 30 leaves by the same side. Therefore, when the contact unit is operated there are two magnetic-forces of attraction acting in parallel upon the disc, that due to the flux' bridging the gap between the disc 30 and the pole face 44, and that due to the flux bridging the gap between the disc 30 and the yoke 67. The yoke 67 is situated slightly beneath the level of the pole face 44 so that the pole face prevents the disc 30 from ever touching it.

The fixed contacts of each of the three constructions of contact unit described above with reference to FIGS. 4, 5, and 6 have been constructed by boring a shallow hole in the end face of a rod so as to leave an annular surface which is then faced with another metal so as to improve its contact-making properties. An alternative construction is provided by the use of fixed contacts which will now be described with reference to FIG. 7, and which are designed for welding to the plane unrelieved end surfaces of the rods. I

Referring now to FIG. 7, the alternative form of fixed contact is provided by a disc of bimetal consisting of a thin layer 70 of gold, copper, silver, silver cadmium oxide, or other material having good electrical contact properties secured to a layer 71 of weldable magnetic material such as permalloy B. The side of the disc having the layer 70 is formed with a shallow circular recess 72 so as to leave an annular contact surface 73. The opposite side is formed with an annular weld stop 74 within the bounds of which the surface is contoured to provide a central projection 75 whose material will be accommodated in the surrounding recess 76 during a subsequent welding process. The welding of this type of contact can then be effected without damaging the annular contact surface '13 so applying a welding tool solely to the recessed portion 72.

These contact units all possess the feature that they can readily be made in an hermetically sealed form. Although they include in their construction glass-to-metal seals, the constructionis such that the seals can be made as one of the first steps of manufacture so that the relatively high temperature involved in their making does not affect the diaphragm which can be added at a later stage. The final sealing operation can then be the sealing of one metal component to another, and can be effected by a method such as projection welding which need heat the bulk of the contact unit scarcely at all. Any of the contact units described above may be linked in a magnetic circuit with an electromagnet so as to form a relay. For this purpose it is convenient to have the operating winding disposed around a part of the inner magnetic member projecting from the contact unit. Except for the provision of relay contact terminals the same basic construction of relay now to be described with reference to FIG. 8a is applicable to each of the three basic types of contact unit, and by way of example only it is the third type of contact unit which is illustrated in this Figure.

Referring now to FIG. 8a the rod 40 forming the inner magnetic member of a contact unit 80 as described with reference to FIGS. 3 and 6 extends substantially the whole length of the relay, and is surrounded by an operating winding 81 wound on an insulating former 82. The rod 40 is covered with an electrically insulating sleeve 83, and fits inside a mild steel tube 84 to which is fixed an end plate 85. The coil former 82 has a flat portion at its base through which protrude the winding terminals 86 of the relay. The three contact terminals 87, respectively secured to the rod 40, the outer magnetic member 42 of the contact unit 80, and the inner cap member 50, protrude in the same direction so that the whole assembly may be directly mounted on a circuit board if desired. Insulating washers 88 are provided to prevent accidental short circuit of these terminals, while the winding 81 is protected by an arcuate cover 89 of magnetic material sprung over the former 82 to grip the end plate 85 at one end and the outer magnetic member of the contact unit at the other. The cover 89 also serves to complete the magnetic circuit so that flux generated by the energization of the winding 81 threads the rod 40, the diaphragm and outer magnetic member of the contact unit 80, returning by the cover 89, the end plate and tube 84. The function of the sleeve 83 is to prevent the short circuiting of the make and moving contacts of the relay without introducing excessive additional reluctance into the magnetic circuit.

An alternative construction of relay is depicted in FIG. 8b which is similar to that described above with reference to FIG. 8a except that the end cap 85 is dispensed with and a contact unit 80 is provided at each end of the operating winding 81. In this relay the cover 89 serves as a magnetic link between the two outer magnetic members of the contact units 80, while the tube 84 serves as the link between the two inner magnetic members provided by rods 40. The contact units 80 may be of the same type or different.

A further alternative construction of relay will now be described with reference to FIGS. 9, 10a and 10b These Figures relate to a polarized magnetically holding relay. For this purpose a permanent magnet is inserted between the tube 84 and the end plate 85, and a magnetic shunt member 91 of magnetic material having the form of the sector of an annulus provides a magnetic shunt bypassing the contact unit 80 so that a substantial proportion of the flux does not thread the unit.

In this construction therefore there is a principal magnetic circuit in which magnetic flux threads the permanent magnet 90, the tube 84, the rod 40, the diaphragm and outer magnetic member of the contact unit 80, returning to themagnet via the cover 89 and the end plate 85. The properties of this circuit are modified by the existence of the shunt member 91 whose outer diameter is just less than the internal diameter of the cover 89. This shunt member is rigidly secured to the tube 84 so that if desired, during manufacture, the assembly of collar, tube, and permanent magnet may be rotated on the sleeve 83.

Referring now to FIGS. 10a and 10b which show a traverse section in the plane A-A of FIG. 9, it will be observed that the reluctance of the shunt path provided by the shunt member 9] depends upon its orientation. FIGS. 10a and 10b showing it respectively in positions of maximum and minimum reluctance. The shunt member 91, the tube 84, the permanent magnet 90 and end plate 85 are secured to each other to form a single assembly which, during manufacture of the relay, can be rotated so as to adjust the reluctance provided by this shunt path in order to set to a desired value the flux threading the contact unit 80. After making this adjustment the end cap 85 may then be cemented to the cover 89 to retain it in the desired position. It is preferred to design the magnetic components so that when the diaphragm is in its released position and the operating winding is not energized there is more magnetic flux threading the shunt than threading the diaphragm.

In this construction the use of a shunt provided by the collar 91 to form a subsidiary magnetic circuit including the permanent magnet but bypassing the contact unit 80 and the part of the rod 40 lying within the operating winding 81 gives rise to a number of advantages. Firstly, during manufacture of the relay it is possible to adjust the value of the flux threading the diaphragm. Secondly, it shunts the permanent magnet 90 from the demagnetizing field produced when the operating winding is energized in such a way as to reduce the flux threading the diaphragm. Thirdly, it lowers the reluctance of the magnetic circuit seen by the operating winding thereby enabling a smaller number of ampere-tums to cause operation or release of the relay. And fourthly, the magnetic holding properties are improved because the difference in the amount of flux threading the diaphragm when it is in its operated position and the amount when it is in its released position is greater by virtue of the fact that some of the flux is diverted from threading the shunt to threading the diaphragm.

Any of the contact units described with reference to FIGS. 4, 5, 6 and 7 finds an alternative application as the contact unit of a switch. For this purpose the contact unit forms part of a magnetic circuit which includes a permanent magnet and contains a magnetic switching member moveable relative to the contact unit in such a way as to alter the amount of flux threading the contact unit's diaphragm. In one form of pushbutton to be described, the switching member is provided by the permanent magnet which is attached to the pushbutton. In another form the switching member is still provided by the permanent magnet, but it is the contact unit which is attached to the pushbutton. In yet another form the permanent magnet and contact unit are fastened together, and the switching member is provided by a magnetic shunt which is normally positioned so that the flux is enabled to bypass the contact unit.

The contact units are particularly suitable for incorporation in miniature pushbutton switches since a practical realization of such a contact capsule need be no larger than 1 cm. in diameter and 0.5 cm. in depth whose contacts are protected from contamination by the environment in which the switch is located.

Four embodiments of pushbutton switches will now be described with reference to FIGS. 11, 12, 13, and 14 which show switches employing contact units of the type described with reference to FIG. 4 (or FIGS. 4 and 7). They can of course be modified to suit the other types of contact unit described above. In each of these embodiments switching is effected by the movement of a permanent magnet relative to the contact unit thereby changing the reluctance of a magnetic circuit including the magnet and the contact unit and hence changing the amount of magnetic flux threading its diaphragm.

FIG. 11 shows a moving magnet pushbutton switch having a flanged hollow pushbutton 101 which is retained by a cooperating flange 102 on a switch housing member 103 surrounding the contact unit104. A cylindrical permanent magnet 105 is secured in the hollow of the pushbutton and is surrounded by a helical compression spring 106 which bears on an insulating washer 107 which surrounds the inner magnetic member 108 of the-contact unit where it emerges from the glass-to-metal seal. Terminal connections to the switch are shown at 109, and the switch assembly is completed by the attachment of a base plate .110 to the switch housing member 103.

The pushbutton 101 is made of magnetic material, but the switch housing member 103 is not, so that magnetic flux from the permanent magnet 105 threads the inner and outer magnetic members of the contact unit 104 and returns to the magnet via the pushbutton 101. The clearance between the magnet and the inner magnetic member is made slightly greater than that between the flange on the pushbutton and the contact unit, so that the magnet is prevented from coming into intimate contact with the inner magnetic member. The strength of the spring is chosen so that it will overcome the full magnetic force of attraction existing when the pushbutton is fully depressed in order that, when released, it will spring back of its own accord. The magnetic force of attraction will of course increase as the pushbutton is depressed, and so can give rise to a feeling of snap-action in operation, which is often desirable in pushbutton switches. A modification of this switch is illustrated in FIG. 12 in which the permanent magnet is not secured to the button but merely held captive by it. The modified pushbutton includes a nonmagnetic flanged tube 111 within which the permanent magnet is permitted a limited amount of free movement in order to provide an enhanced snap-action associated with the operation of the switch. As before, even when the pushbutton is fully depressed, the clearances may be chosen so as to prevent intimate contact between the magnet and the inner magnet 108. A nonmagnetic washer 112 prevents direct contact between the permanent magnet and the magnetic part of the pushbutton. There is now seen to be a gap in the magnetic circuit at each end of the permanent magnet and the dimensions of the switch can be chosen so that when the pushbutton is depressed the magnetic attraction across the gap between the magnet and the inner magnetic member rises to exceed that across the washer 112. When this happens, the magnet snaps against the retaining flange of the tube 111. If the dimensions have been correctly chosen, as the pushbutton is released the flange of the tube 111 will carry the magnet away from the inner magnetic member until a point is reached when the attractive force across this widening gap is reduced below that across the gap at the other end of the magnet, whereupon the magnet will snap back to its original position resting against the washer 1 12.

FIG. 13 illustrates how by means of an extra flange, indicated at 131, on the housing member 103 and dished washer 132 the switch can be adapted to have provision for overtravel of the pushbutton allowing it to be depressed further than is required to bring the magnet to its closestapproach to the inner magnetic member of the contact unit.

In the foregoing examples of pushbutton switches, although the contacts are completely sealed within the contact unit, there is a slot in the side of the pushbutton to accommodate the terminal connection to the inner magnetic member, and it may be possible for foreign material to enter the switch through this opening. FIG. 14 shows an alternative construction of pushbutton switch in which the contact unit 104 is attached to the pushbutton 101 and the permanent magnet 105 is secured to the base plate 110. With this construction it is possible to fit a gasket between the cooperating flanges of the pushbutton 101 and the housing member 103 so as to seal the interior of the switch from contamination from the outside. This gasket is of course less effective while the pushbutton is depressed. In place of the single compression spring there are two concentric compression springs 141 and 142, which also form the electrical connections to the inner and outer magnetic member of the contact unit and are connected to terminal pins 143 located in insulating sleeves 144 in the baseplate 110. Connection to the inner magnetic member 108 is made via an electrically conductive washer 145. An insulating sleeve 146 around the permanent magnet protects the inner compression spring from an accidental short to the magnet. In distinction from the former pushbutton switches it is the housing member 103 which is made of magnetic material and the pushbutton 101 which is made of nonmagnetic material.

The above-described embodiments of pushbutton switches have all been of the type in which operation is effected by the movement of a permanent magnet relative to the contact unit so as to alter the reluctance of the magnetic circuit which includes the magnet and the contact unit. In alternative embodiments the permanent magnet can be fixed in relation to the contact unit and can even form part of the unit, the reluctance of the circuit being altered by the movement of some magnetic part which is not a permanent magnet.

In another form of construction illustrated in FIG. 15 the movement of the pushbutton does not change the reluctance of the magnetic circuit which includes the permanent magnet and the contact unit, but changes the reluctance of a magnetic shunt path by passing the contact unit.

Referring to FIG. 15 the permanent magnet 105 in this construction is attached to the contact unit 104 via a disc 151 of magnetic material and an insulating molding 152 surrounding the inner magnetic member 108 the contact unit is housed within the housing member 103, which is made of nonmagnetic material, and both are attached to the base plate 110. The opposite end of the permanent magnet flts in a cap 153 of magnetic material which is a sliding fit inside a nylon liner 154 lining the inside of the pushbutton 101. This liner 154 is introduced to reduce the friction between the cap and the pushbutton. The pushbutton, which is made of magnetic material, is formed with a flange 155 which cooperates with a flange formed on the housing member so that the pushbutton is held captive. Underlying this flange 155 is a washer 156 of mag netic material which is normally attracted by the permanent magnet into contact with the disc 151. This disc is formed within a radial slot to accommodate the terminal connection 109 to the inner magnetic member and is insulated from that terminal by part of the moulding 152. The washer and the disc together form a magnetic shunt by passing the contact unit. When the pushbutton is depressed the reluctance of this shunt path is increased and therefore more flux is diverted into threading the contact unit. Provided that a strong enough permanent magnet is used this pushbutton switch does not need a return spring for the pushbutton because there is sufficient magnetic restoring force to cause the pushbutton to rise when it is released. The sole magnetic restoring force is provided by the attraction across the gap between the washer 156 and the disc 151, and therefore, if a return spring is safely to be dispensed with, the permanent magnet has to generate sufficient magnetic flux to operate the contact unit when the pushbutton is fully depressed and still leave sufficient flux threading this gap to provide a stronger force of attraction across it than any counteracting forces. The counteracting forces are those acting on the washer 156 due to the flux threading the contact unit, and that acting directly on the pushbutton due to any flux threading the gap between the magnet and the top of the pushbutton. This last force is reduced to a minimum by the provision of the cap 153 which diverts the great majority of the flux direct to the sidewalls of the pushbutton.

Although in the foregoing description the method of operation of the switch contacts has been by means of a change of reluctance of a magnetic circuit associated with the magnet and contact unit, this is not the only method available. An alternative method of switching is possible in which the flux change in the contact unit is caused by a change of magnetomotive force in its magnetic circuit. This can be effected, for example, by rotation of the permanent magnet so that its direction of magnetization can either lie in the magnetic circuit thereby giving rise to maximum m.m.f., or across it, in which case the m.m.f. drops to zero.

It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitation on its scope.

What is claimed is:

l. A magnetically responsive switching unit comprising a magnetic circuit including in series an inner member, a diaphragm, and an outer member, means spacing said inner member from said outer member, said diaphragm comprising a disc with three or more arcuate tongues spaced annularly about the periphery of said disc and each disposed with one end of the tongue extending freely from the body of the disc, said outer member comprising a fixed structure cooperatively associated with said tongues for completing an electrical circuit therethrough, wherein said diaphragm is responsive to completion of said magnetic circuit to move resiliently toward said inner member on operation of said switch, wherein each of said tongues terminates at its free end in a U-shaped contact member substantially normal to the disc, and said fixed surface comprises one extremity of said outer member.

2. The switching unit of claim 1, in which said diaphragm is axially spaced from said inner member by the spacing of said U-shaped members with the switch in an open circuit condition.

3. The switching unit of claim 2, wherein said inner member comprises an electrically conductive rod positioned to complete an electrical circuit to said diaphragm with the switch in a completed magnetic circuit condition.

4. A magnetically responsive switching unit comprising an outer annulus, a magnetic rod, means for spacing said rod coaxially within said annulus in a fixed relationship and a magnetically responsive disc coaxially mounted to substantially bridge the space between said annulus and said rod,

a plurality of tongues angularly spaced about the disc periphery within the plane of said disc in an outer ring thereof,

depending contacting members at the free end of each tongue cooperatively engaging a surface of said annulus,

means for completing a magnetic field between said annulus, said rod and said diaphragm to produce movement of said diaphragm, and said diaphragm is magnetically responsive to said field completion to move toward said rod and thereby switch an electrical circuit between said diaphragm and rod.

5. A switching unit as claimed in claim 4, wherein the free ends of said plurality of tongues are separated from the disc body by a substantially annular discontinuous slit, whereby to enhance the inherent resiliency of said diaphragm.

6. A switching unit as claimed in claim 4, wherein each of said tongues comprise a U-shaped section extending substantially normal to the disc, the height of said sections spacing said diaphragm from said road in an open circuit condition. 

1. A magnetically responsive switching unit comprising, a magnetic circuit including in series an inner member, a diaphragm, and an outer member, means spacing said inner member from said outer member, said diaphragm comprising a disc with three or more arcuate tongues spaced annularly about the periphery of said disc and each disposed with one end of the tongue extending freely from the body of the disc, said outer member comprising a fixed structure cooperatively associated with said tongues for completing an electrical circuit therethrough, wherein said diaphragm is responsive to completion of said magnetic circuit to move resiliently toward said inner member on operation of said switch, wherein each of said tongues terminates at its free end in a U-shaped contact member substantially normal to the disc, and said fixed surface comprises one extremity of said outer member.
 2. The switching unit of claim 1, in which said diaphragm is axially spaced from said inner member by the spacing of said U-shaped members with the switch in an open circuit condition.
 3. The switching unit of claim 2, wherein said inner member comprises an electrically conductive rod positioned to complete an electrical circuit to said diaphragm with the switch in a completed magnetic circuit condition.
 4. A magnetically responsive switching unit comprising an outer annulus, a magnetic rod, means for spacing said rod coaxially within said annulus in a fixed relationship and a magnetically responsive disc coaxially mounted to substantially bridge the space between said annulus and said rod, a plurality of tongues angularly spaced about the disc periphery within the plane of said disc in an outer ring thereof, depending contacting members at the free end of each tongue cooperatively engaging a surface of said annulus, means for completing a magnetic field between said annulus, said rod and said diaphragm to produce movement of said diaphragm, and said diaphragm is magnetically responsive to said field completion to move toward said rod and thereby switch an electrical circuit between said diaphragm and rod.
 5. A switching unit as claimed in claim 4, wherein the free ends of said plurality of tongues are separated from the disc body by a substantially annular discontinuous slit, whereby to enhance the inherent resiliency of said diaphragm.
 6. A switching unit as claimed in claim 4, wherein each of said tongues comprise a U-shaped section extending substantially normal to the disc, the height of said sections spacing said diaphragm from said roaD in an open circuit condition. 